1. Field of the Invention
The present invention relates to electron beam lithography and more particularly, to a direct patterning method of a resist film using an electron beam, which is applicable for semiconductor device fabrication such as circuit pattern formation in an electron resist film placed on or over a semiconductor substrate.
2. Description of the Prior Art
In recent years, a great variety of semiconductor integrated circuit devices each of which is fabricated in a small amount, such as custom large-scale integrated circuit devices (LSIs) and semicustom LSIs, have been increasingly demanded designed and produced.
To fabricate such devices as above, a direct writing or drawing method of a resist film placed on or over a semiconductor wafer using an electron beam has been developed and employed. With this method, a mask and a reticle as required for the conventional projection exposure method are not needed and desired patterns are directly written in the resist film according to the pattern data generated in a pattern generator. As a result, the direct writing method offers an advantage that the fabrication term of the semiconductor device is shortened and that the fabrication cost thereof is reduced, both of which are due to the lack of a mask and reticle.
A conventional direct patterning method of this sort is described below referring to FIGS. 1A and 1B, which is carried out by a conventional scanning electron-beam exposure system.
To accomplish the semiconductor devices such as LSIs on a semiconductor wafer, in general, resist-patterning processes are repeated several or several tens times for the same wafer. In other words, a lot of patterns are written sequentially in corresponding resist films for the same wafer. Therefore, it is very important for all the given patterns to be overlaid accurately. Otherwise the devices on the wafer will not meet the performance specifications.
Accordingly, several alignment marks, which may be etched trenches, metal layers or the like, are formed on or over the surface of the semiconductor wafer in advance. An electron beam is irradiated to scan an exposure field of the wafer prior to each pattern writing process in order to recognize the marks. Then, the beam is scanned again over the exposure field by reference to the marks in order to write corresponding one of the given patterns.
Specifically, as shown in FIG. 1A, a silicon (Si) wafer 301 has an alignment mark 305 formed by an etched trench in an exposure field of the wafer 301. On the surface of the wafer 301, a silicon dioxide (SiO.sub.2) film 302 with a thickness of 0.5 to 2 .mu.m, an aluminum (Al) fill 303 with a thickness of 0.5 to 1.5 .mu.m, and an electron resist film 304 with a thickness of 2.1 to 2.5 .mu.m are formed in this order.
In the case of patterning the Al film 303, an electron beam EB produced by an electron gun is irradiated to the exposure field of the semiconductor wafer 301 held on a wafer table, and is scanned along the direction W to cross the alignment mark 305.
The acceleration voltage of the incident electrons of the beam EB is typically 20 kV to 50 kV. When the acceleration voltage is lower than 20 kV, the electron beam EB fluctuates in electron density at the spot formed on the resist fill 304 so that the electron density decreases on average. Thus, the pattern writing process takes a longer time than the came of 20 kV to 50 kV because of an increased interaction time between the incident electrons and the resist film.
In addition, small patterns cannot be written because the incident electrons are affected by their aberration.
On the other hand, when the acceleration voltage is higher than 50 kV, a part of the incident electrons of the beam EB penetrate the resist film 304, so that the resist film 304 decrease in sensitivity, i.e., the number of the electrons that contribute the interaction with the resist film reduces. As a result, also in the case, the pattern writing process takes a longer time than the case of 20 kV to 50 kV.
Almost all the incident electrons of the beam EB thus irradiated pass through the resist film 304, the Al film 303 and the SiO.sub.2 film 302 and then strike the surface of the wafer 301 or the bottom face of the mark 305. The incident electrons thus struck are reflected by the surface of the wafer 301 or the bottom face of the mark 305 to be back-scattered electrons 53, respectively. The reference 52 indicates secondary electrons generated simultaneously with the secondary electrons 53 at the striking area of the mark 305. The secondary electrons 52 will disappear in the vicinity of the striking area because they have a kinetic energy as low as 100 eV or less.
The back-scattered electrons 53 pass through the SiO.sub.2 film 302 to strike the back surface of the Al film 303, so that secondary electrons 54 are generated at the striking positions or areas to enter the Al film 303, respectively.
The back-scattered electrons 53 have high kinetic energies, almost the same as those of the incident electrons of the beam EB. Therefore, the electrons 53 can pass through all of the SiO.sub.2, Al and resist films 302, 303 and 304 to go out of the resist film 304.
To recognize the alignment mark 305 during the scan, an electron detector of the scanning electron-beam exposure system continues to detect the back-scattered electrons 53 to produce an electric signal as a function of position on the surface of the wafer 301.
As shown in FIG. 1B, the signal produced by the back-scattered electrons 53 varies in amplitude at the alignment mark 305, so that the position of the mark 305 can be recognized from the signal. Then, a corresponding one of the pattern writing processes starts. The acceleration voltage of the beam EB for each writing process is equal to that for the mark-recognition process.
The back-scattered electrons 51 and the secondary electrons 55, which are generated at the surface of the resist film 304, are also detected by the electron detector, so that they produce noise on the signal.
With the conventional directly patterning method, when the total thickness of the SiO.sub.2, Al and resist films 302, 303, and 304 is comparatively small, the signal produced by the back-scattered electrons 53 has a sufficiently large amplitude compared with the noise produced by the back-scattered and secondary electrons 51 and 55. As a result, all the given patterns can be written to be overlaid accurately in the resist film 304.
However, when the total thickness of the SiO.sub.2, Al and resist films 302, 303, and 304 is comparatively large, the incident electrons of the electron beam EB tend to lose their kinetic energies partially or entirely before they reach the wafer 301 or the alignment mark 305. Therefore, almost all the back-scattered electrons 53 reflected by the surface of the wafer 301 or the bottom face of the mark 305 do not have the energies sufficient for passing through the films 302, 303 and 304 to go out of the film 304.
For example, the number of the back-scattered electrons 53 detected outside the resist film 304 decreases to one tenth ( 1/10) of that of the electrons 53 generated at the surface of the wafer 301 or the bottom face of the mark 305.
Thus, the electric signal produced by the back-scattered electrons 53 decreases in amplitude so that the signal cannot be distinguished from the noise, as shown in FIG. 1B, reducing the signal to noise ratio (S/N). As a result, there is a problem that the patterns written in the resist film 304 are not overlaid on each other exactly due to reduction of alignment accuracy, which leads to deterioration of fabrication yield.